The limited capacity of power sources, ie. batteries, and their relatively short operating time between charging periods is a known problem with mobile telephones. In an attempt to make the operating time, ie. the time between chargings, of a mobile telephone longer, all circuits of the telephone are switched off when they are not used. However, some parts of the telephone's power supply circuitry are continuously connected to the battery and, therefore, in present devices, there is a continuous discharge current of about 400 to 500 .mu.A from the battery. In addition, there is the self-discharge current of about 100 .mu.A discharging from the battery. When a battery pack with the capacity of, say, 400 mAh is attached to the telephone, it will discharge in about 28 days even if the telephone is not used (400 mAh/(500 g A+100 .mu.A)=667 h=28 days).
One of the parts in the power supply circuitry is a voltage comparator which has to be capable of accurate detection of the input voltage, ie. voltage comparison. Then the voltage of the analog input pin has to be detected very accurately to start a certain integrated circuit and then also the threshold value of the input signal detector has to be very accurate. The device should not function without this certain voltage in the pin and, on the other hand, the device should be turned on immediately after this certain voltage becomes available.
Appropriate prior art comparison circuits will be studied with the help of an example, with reference to the enclosed FIGS. 1 and 2, of which FIG. 1 shows an accurate detection method, or a comparison circuit, for an input signal and Figure its signals as a function of the input signal voltage level. FIG. 1 shows a comparator 1 which compares the input signal VIN to an accurate reference voltage VREF. The comparator 1 switches the output voltage VOUT on when the input voltage VIN exceeds the reference voltage VREF. FIG. 2 shows signals of the accurate detection of input signal as a function of the input signal voltage level. In the drawing, the input voltage VIN is represented by curve 2 and the output voltage VOUT by curve 3. The comparator 1 switches the output voltage VOUT on when the input voltage VIN exceeds the reference voltage VREF.
When a mobile telephone uses for voltage detection a method like the one described above, the discharge current of the battery is far too big for a lithium battery. It would be interesting to use a lithium battery but, depending on the case, a lithium battery can be continuously discharged with a current of 10 to 100 .mu.A, at the most. In prior art solutions, the use of lithium batteries is not possible without a hardware switch or a similar circuitry.